example ipv6 outgoing multicast interface

On most systems, where scope format uses the interface name:

const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
  socket.setMulticastInterface('::%eth1');
});

On Windows, where scope format uses an interface number:

const socket = dgram.createSocket('udp6');
socket.bind(1234, () => {
  socket.setMulticastInterface('::%2');
});